2.1. GENERAL CONSIDERATIONS
Polyfunctional macromolecules, such as proteins, can be purified by a variety of techniques. One of these techniques is known as ion-exchange chromatography. In ion-exchange chromatography, proteins are separated on the basis of their net charge. For instance, if a protein has a net positive charge at pH 7 it will bind to a negatively charged ion-exchange resin packed in a chromatography column. The protein can be released, for example, by decreasing the pH or adding cations that compete for binding to the column with the positively charged groups on the protein. Thus, proteins that have a low density of net positive charge, and thus a lower affinity for the negatively charged groups of the column, will tend to emerge first, followed by those having a higher charge density.
Generally, the ion-exchange resins which are used in these procedures are solids possessing ionizable chemical groups. Two types exist: cation-exchangers, which contain acidic functional groups such as sulfate, sulfonate, phosphate or carboxylate, and a second type, anion-exchangers, which contain functional groups such as tertiary and quaternary amines. These ionizable functional groups may be inherently present in the resin or they may be the result of the chemical modification of the organic or mineral solid support.
Organic ionic-exchangers which are made from polysaccharide derivatives, e.g., derivatives of agarose, dextran and cellulose, etc., have been used for both laboratory and industrial scale ion-exchange chromatography. However, these ion-exchangers have many disadvantages. First, polysaccharide-derived ion-exchangers are not very mechanically stable and are not resistant to strong acids. This instability limits the length of the column and, also, limits the flow rate through the column.
Second, such ion-exchangers have limited sorption capacity due to the limited number of ionic or ionizable groups that can be attached to the polysaccharide.
Third, these polysaccharidic derivatives are poor adsorbents for use in rapid fluidized-bed separations because of the low density of the material. In a fluidized bed it is desirable to pass the fluid without simultaneously washing out the particles. Therefore, it is generally desirable to have as great a density difference as possible between the solid support particles (e.g., silica) and the fluidizing medium.
The intrinsic high density of inorganic sorbents based on passivated mineral substrates facilitates packing and rapid decantation into chromatographic columns. Dense packing prevents formation of empty spaces and channeling when using packed beds. On the other hand, fluidization of dense particles in aqueous suspension is possible at high flow rates that, in turn, are very desirable when dealing with large scale applications. Operation of fluidized beds at high superficial flow velocities is generally not possible with low-density organic or polymeric sorbents, which can be elutriated from fluidized beds at relatively low liquid flow rates.
On the other hand, synthetic polymers are mechanically more stable than inorganic supports, and the former are more resistant to strong acidic conditions. However, they suffer disadvantages as well, such as limited capacity, limited solute diffusivity and thus, limited productivity. These synthetic polymers also suffer to some extent from the problem of non-specific adsorption of biomolecules, such as proteins. Untreated mineral supports such as silica are also inadequate in many chromatographic protein separation applications because of such non-specific adsorption.
Non-specific adsorption is caused by the interaction of a protein with the surface of the support--be it organic or inorganic in nature. For example, silica is an acidic compound, and the negatively charged silanol groups present at the solid/liquid interface tend to create a separate ion-exchange interaction between the surface of silica and the protein. Non-specific adsorption is also caused by hydrogen bonding that takes place between, e.g., amino groups present in the amino acid residues of proteins and these same silanols present at the silica surface. Such non-specific interactions create separation problems during chromatography--e.g., poor protein recovery and/or inadequate resolution. An important objective in the design of a chromatographic separation is generally to ensure a "single-mode" process of adsorption. However, the ion-exchange behavior associated with surface silanols can create a "mixed mode" adsorption system which makes the separation of biomolecules much more difficult. Although the sorption capacity generated by ionic silanol groups is low, the intensity of the interaction between the silanol groups and proteins can be high. These interactions therefore have the potential to cause denaturation of certain proteins.
Finally, both polysaccharides and most hydroxyl-containing synthetic sorbents are sensitive to the cleaning solutions used in industrial settings, which often include strong oxidizing agents such as hypochlorite or peracetic acid and which may be characterized by extremes of pH.
Thus, there is an important need for the development of improved passivation methods for the treatment of the surfaces of both polymeric and inorganic chromatographic supports in contact with protein-containing solutions, which method is capable of preventing or minimizing such non-specific interactions between proteins and the chromatographic support in order to improve the efficiency of chromatographic processes.
2.2. PREVIOUS EFFORTS AT COATING SOLID SUPPORTS
Several previous investigators have sought to passivate various microporous media including membranes and particulate chromatographic supports by applying thin surface coatings to inorganic or organic/polymeric substrates. For example, Steuck, in U.S. Pat. No. 4,618,533, discloses a porous polymeric membrane substrate fashioned from a thermoplastic organic polymer upon which a permanent coating is grafted and/or deposited on the entire membrane surface. The polymerization and crosslinking of the polymerizable monomer upon and within the porous membrane substrate is performed in such a way that a thin coating is deposited upon the entire surface of the porous membrane, including the inner pore walls. Significantly, the porous configurations of the coated, composite membrane structures claimed by Steuck are essentially identical to those of the corresponding uncoated porous membrane substrates, implying that the polymer of Steuck is applied as a thin surface layer or coating that does not interfere with the porosity or flow properties of his composite membranes. Moreover, Steuck does not disclose the concept of a "passivating layer" or the use of monomers capable of functioning as "passivating" monomers within the meaning of the present invention as discussed in more detail below.
Varady et al., in U.S. Pat. No. 5,030,352, disclose pellicular support materials useful as chromatography media which are obtained by applying various thin hydrophilic coatings to the surfaces of hydrophobic polymer substrates (e.g., polystyrene). Varady's surface coatings are applied by first exposing the surfaces of the hydrophobic substrate to a solution of a solute characterized by interspersed hydrophilic and hydrophobic domains; contact between surface and solute takes place under conditions that promote hydrophobic-hydrophobic interaction between solute and substrate, with the result that solute molecules are adsorbed onto the surface of the substrate as a thin coating that is ultimately crosslinked in place. Varady's coating materials may further comprise reactive groups capable of being derivatized to produce various materials useful in ion-exchange, affinity, and other types of chromatographic and adsorptive separations.
Significantly, however, the hydrophilic, functional coating of Varady's invention is limited to a thin adherent film on the surface of the hydrophobic support. The morphology of this coating layer is a direct and unavoidable consequence of the stated method of its deposition--i.e., by the crosslinking of adjacent solute molecules adsorbed onto the surface of the hydrophobic substrate.
While Varady's coating method is at least partially effective in reducing the non-specific binding of proteins to the substrate, the sorption capacity of the chromatographic materials so produced is necessarily limited and inferior to those of the media produced by the process of the present invention. As discussed in considerably more detail below, the method of the present invention causes the formation of a crosslinked and functional gel that extends out into and substantially fills the pores of the support. As a consequence, the static and dynamic sorption capacities of the chromatographic media are not limited by the porous surface area of the substrate, as is the case with the pellicular materials of Varady's invention.
With regard to previous techniques for the passivation of inorganic or mineral supports by surface coating treatments, U.S. Pat. No. 4,415,631 to Schutijser discloses a resin consisting of inorganic silanized particles onto which is bonded a crosslinked polymer comprised of copolymerized vinyl monomers and which contains amide groups. The invention specifies that the inorganic porous support, including silica, must be silanized prior to coating. The silanization treatment provides the inorganic porous support with reactive groups so that the copolymer can be covalently bonded to the silica surface.
Nakashima et al., in U.S. Pat. No. 4,352,884, also discloses the use of silica as a porous substrate. The silica is coated with a polymer made up of acrylate or methacrylate monomer and a copolymerizable unsaturated carboxylic acid or a copolymerizable unsaturated amine. Nakashima et al. use an already preformed polymer to coat the support. Furthermore, Nakashima et al., in a separate and distinct step, utilize a crosslinking agent in a subsequent curing process.
The above-mentioned inventions are not completely successful, partly because of the unstable chemical linkage between the silica and the coating. The products of these inventions have the further disadvantages of not only failing to totally suppress the initial non-specific adsorption but also of introducing additional modes of non-specific adsorption.
Tayot et al., in U.S. Pat. No. 4,673,734, disclose a porous mineral support that is impregnated with an aminated polysaccharide polymer that is said to cover the internal surface area of the support. However, since polysaccharides usually have very large molecular weights and their solutions are quite viscous, this process is not highly effective. Coverage of the entire internal surface of the silica substrate is problematic due to incomplete and uneven filling of the pores of the silica substrate by the large polysaccharide molecules.
The steric problems of Tayot's process result from the large size of the polysaccharides employed, the chains of which cannot penetrate completely within the pores of the support. This incomplete penetration results in the creation of a "soft" layer of polysaccharide on the surface of the pore that subsequently causes problems during chromatographic separation. Polysaccharides such as dextran can also spontaneously hydrolyze at low pH, rendering them incompatible with certain cleaning operations that require the column or bed of chromatographic media to be washed with acid, alkaline, or oxidizing agents.
Despite these and other problems associated with the use of inorganic chromatographic supports, the use of mineral compounds such as silica as supports for chromatographic adsorbents is still attractive, because as explained above, chromatographic separations can be performed with such materials at very high flow rates--for example, in very large-scale packed columns or in fluidized beds for industrial operations. What is needed are chromatographic supports characterized by high static and dynamic sorption capacity which exhibit improved chemical stability at alkaline and basic conditions and reduced tendencies to cause non-specific protein adsorption. It is an object of the present invention to provide such supports.